In a SISO (single input single output) OFDM (orthogonal frequency division multiplexing) system, i.e. a communication system with one transmit antenna and one receiver antenna which uses modulation of a plurality of subcarriers according to OFDM for data transmission, preambles are sent before the user data is sent for various reasons. First, short preambles are sent for timing synchronisation and frequency synchronisation, in particular for frequency offset estimation.
Frequency offset estimation is necessary since in practical data communication, it cannot be expected that the local oscillator frequency at the receiver is identical to that of the signal carrier generated at the transmitter. This is for one due to circuit limitation and it particularly arises if the receiver is in relative motion to the transmitter as Doppler shift is inevitably introduced to the carrier frequency.
A frequency offset may lead to inter-carrier interference (ICI). For a multicarrier system, such as one that adopts OFDM, a residual frequency offset results in a significant performance degradation.
After the short preambles, relatively few long preambles are sent in a SISO OFDM system (two in case of a system according to the IEEE802.11a standard, see [1]) which are employed for channel estimation. Since only a single channel is established, few long preambles are enough for channel estimation.
In case of a MIMO (Multiple Input Multiple Output) system where a plurality of transmit antennas are used, however, a higher number of long preambles is necessary in order for all channel information to be extracted since a plurality of physical channels are employed for transmission. In fact, it can be shown that in case of a MIMO system, the number of long preambles should be no less than the number of transmit antennas.
Such an increase of the number of long preambles compared to a SISO system has the consequence that the time for transmission of the long preambles, also called the long preamble duration, is increased compared to a SISO system. This gives rise to a more substantial rotation of the phase in the data symbols succeeding the long preambles in the presence of a residual frequency offset which is involuntary due to the finite number of short preambles for frequency offset estimation (FOE) in a practical communication system.
Due to the severity of the distortion, the data symbols sent first can typically not be adequately corrected and error bits and error packets are caused.
In [2] an estimator for frequency estimation is described.
An object of the invention is to provide a method for improved residual frequency offset estimation in communication systems.
The object is achieved by a method for determining a residual frequency offset, a communication system, a method for transmitting a message by a transmitter, a transmitter, a method for processing a message and a receiver with the features according to the independent claims.